Why do the (USA) present rules re ELT's stipulate that they have to be
mounted and meet FAA specifications, and that personal ones do not meet
the rules?

If you bale out, you might be a long way from the glider wreckage and
remain undiscovered if rescuers only have a fix on the glider wreckage.
Most survivable accidents, bale out or still in cockpit, have the
pilot still able to push a button and could use a personal ELT. That
seems to be the thinking of several UK piots who have or are thinking
about ELT's. I looked at the installation/certification details of a
fixed one (no approved scheme for it in the UK) and it looks difficult
to the point of being impractical.

Chris N.